Old-growth forests are generally considered to be large groups of trees that have not been disturbed by human intervention like logging. An old-growth forest actually has trees of all ages–from saplings to giants–which are created over lengthy periods of time, from natural disturbance and recovery. At Sempervirens Fund, an old-growth forest is defined as an uneven-aged forest dominated by large, old trees that show signs of age and injury, like large branches, broken-tops, and damaged branches, and that contain these elements: a multi-layered canopy, large snags, and large downed trees.

Complex structure features like these are important in both individual trees and forest-wide. Diversity in tree ages creates a diversity of habitat conditions with multi-layered plant communities which provide food sources for animals and promote fog drip collection. Broken tops, fire scars, basal hollows (goose pens), large, reiterated limbs, and reiterated crowns all provide habitat for plants and animals. Even dead trees are integral in the forest. Large standing dead trees called snags are critical for wildlife like nesting owls and other birds. Large, downed trees, known as “large woody debris” or "coarse woody debris,” are shelter for amphibians, small mammals, reptiles, and invertebrates, and their slow decay on the forest floor releases important nutrients into the ecosystem. Some dead trees end up in streambeds, where they help improve water quality and provide habitat for amphibians and fish like endangered coho salmon to lay eggs. Diversity in tree ages and complex structures that can only develop over long periods of time contribute to increased species richness and abundance in old-growth forests.